Human stem cells, which have been expanded in culture from a small amount of donor cells, can be used to repair or replace damaged or defective tissues and have broad clinical applications for treatment of a wide range of diseases. Recent advances in the area of regenerative medicine demonstrate that stem cells have unique properties such as self-renewal capacity, the ability to maintain the unspecialized state, and the ability to differentiate into specialized cells under particular conditions.
As an important component of regenerative medicine, the bioreactor or cell expansion system plays a role in providing optimized environments for cell growth and expansion. The bioreactor provides nutrients to the cells and removal of metabolites, as well as furnishing a physiochemical environment conducive to cell growth in a closed, sterile system. Cell expansion systems can be used to grow other types of cells as well as stem cells.
Many types of bioreactors are currently available. Two of the most common include flat plate bioreactors and hollow fiber bioreactors. Flat plate bioreactors enable cells to grow on large flat surfaces, while hollow fiber bioreactors enable cells to grow either on the inside or outside of the hollow fibers.
It is not current practice to look inside a bioreactor to determine when to harvest the expanded cells without destroying the sterility of the closed system. A way to determine when to harvest the cells while still maintaining sterility is necessary. It is to such methods that the present invention is directed.